Ultraviolet (“UV”) light is radiation in the wavelength range of 260-400 nm. It is part of the solar radiation that reaches the Earth, and has critical impact on humans. The skin synthesizes Vitamin D on exposure to UV which makes UV necessary for health. But overexposure to UV can cause adverse effects such as sunburn, systemic reactions in autoimmune diseases such as lupus, or pharmaceutical phototoxicity in the short term, and non-melanoma and melanoma skin cancer, skin aging, pharmaceutical photoallergy, photogenotoxicity, and photocarcinogenicity in the longer term (‘adverse effects’ thereafter). Sensitivity to UV varies from person to person, e.g., darker skin types scatter more UV in the top layers of skin and hence are at lower risk for sunburn. However, their skin synthesizes less Vitamin D than lighter skins. Sun-related activity also varies from individual to individual. Outdoor runners are more exposed to UV than indoor treadmill runners. Certain professions, such as construction, involve large exposure to UV on a daily basis, while office jobs involve lower UV exposure. Under such circumstances, the primary way to be able to control any adverse event of overexposure is to have an accurate knowledge of personal UV dose. This is what is achieved by the proposed exemplary systems and methods, where the wearable device measures UV exposure and aggregates it to compute the UV dose, while the mobile device displays metrics and alerts to the user based on this information. In alternative designs, the mobile device, which may be referred to herein as a “remote device,” can process one or more signals and aggregate the information.
How to Measure Solar UV Radiation in a Way Relevant to Human Health?
In 1987, the human sensitivity to ultraviolet radiation was defined by Diffey and later adopted by the World Metereological Organization and the World Health Organization (McKinlay, A. & Diffey, B. “A reference action spectrum for ultra-violet induced erythema in human skin”. CIE J. 17-22 (1987)). This sensitivity is called the erythema action spectrum and gives exponentially more importance to high-energy photons. When measured on a horizontal surface, this standard metric is called the ultraviolet index (UV Index, or UVI).
What impacts human health is the integration of UV exposure over time, referred to herein as the “UV dose.” When the UV exposure is weighted according to the erythema action spectrum, the accumulated dose is called the “erythemal dose.”
Some UV measuring systems include a UV measuring diode, which converts the incident ultraviolet radiation signal to electric current, coupled with additional circuitry. This can include an analog-to-digital converter (ADC), op-amp and microcontroller, such as is described in Amini N., Matthews E. J., Vandatpour A., Dabiri F., Noshadi H., Sarrafzadeh M., “A Wireless Embedded Device for Personalized Ultraviolet Monitoring,” International Conference on Biomedical Electronics and Devices, pp. 200-205 (2009). Some examples of these systems are the Solarmeter® 6.5 UVI and the Genicom UV Index Meter. While such systems might be capable of approximately measuring UV, they are not accurate in a wide variety of situations, as has been reported in Corrêa, M. D. P. et al. “Comparison between UV index measurements performed by research-grade and consumer-products instruments.” Photochem. Photobiol. Sci. 9, 459-463 (2010), and Larason, T. C. & Cromer, C. L. “Sources of error in UV radiation measurements”. J. Res. Natl. Inst. Stand. Technol. 106, 649-656 (2001).
Why is Accuracy Important in UV Measurements?
Several diseases or pharmaceutical treatments are negatively or positively (up to a certain point) impacted by UV exposure. For instance, UV exposure is sometimes used in the treatment of psoriasis and the dose of UV exposure used in these treatments is well defined. On the other hand, going over a threshold of UV dose can trigger symptoms in lupus patients, phototoxicity for certain drugs, or erythema and sunburn of the skin. Some clinical experiments have found the threshold for erythema (Sayre, R. & Desrochers, D. “Skin type, minimal erythema dose (MED), and sunlight acclimatization”. Am. Acad. dermatology 439-443 (1981); Heckman, C. J. et al. “Minimal Erythema Dose (MED) testing”. J. Vis. Exp. e50175 (2013) doi:10.3791/50175) for instance but most UV dose threshold are unknown—and they vary from person to person. Even when UV dose thresholds are known, it is important to know current dose relative to such thresholds. Overestimation of UV dose can lead to less time outside, hindering the capacity of planning properly one's day. Underestimation can lead to longer periods spent while exposed to UV (whether it is in sunlight or in the shade), which can easily cause adverse effects. This makes accuracy of extreme importance in the measurement of UV dose. Since UV dose is the UV exposure integrated over time, it follows that accurately measuring UV dose implies also accurately measuring the UV exposure.
An approximate forecast for the maximum daily UV Index is usually provided by local weather services, but is largely inadequate for measuring personal UV exposure. Whether a person is in the shade, in direct sunlight, in indirect sunlight, this forecast is the same although actual UV exposure varies dramatically.
What are the Advantages of Real-Time Measurement of UV Dose?
The importance of thresholds is already known—whether it is for UV-induced lupus symptoms, pharmaceutical phototoxicity, or erythema and sunburn. If the UV dose threshold is being approached, or has been exceeded, this information needs to be conveyed to the user so that he/she can act on it immediately. Otherwise it can lead to adverse effects that might lead to hospitalization. It is for instance known that UV exposure has a systemic impact on lupus and evidence shows that lupus patients experience more flares in the summer than in the winter, as reported in Chiche, L. et al. Seasonal variations of systemic lupus erythematosus flares in southern France. Eur. J. Intern. Med. 23, 250-254 (2012).
The Importance of Separating UVB from UVA
The clinical literature, whether it is looking at skin cancer, Vitamin D, photosensitivity, phototoxicity, photocarcinogenicity, differentiates between UVB (280-320 nm wavelengths) and UVA (320-400 nm wavelengths). This is because the two types of UV have different impact on the human physiology. The depth of penetration of UV light into the skin increases with increasing wavelength. While UVB is absorbed in the upper layer of the skin, UVA is able to travel further into the skin. For this reason, UVA, although less energetic than UVB, has a significant impact on autoimmune reactions and phototoxicity/photosensitivity. Both UVB and UVA can cause redness of the skin, drug-induced reactions, and trigger the immune system to react. Outside, under solar radiation, UVB rays burn the skin before UVA do. For these reasons, the medical community stresses the importance of differentiating UVB and UVA when a UV dose is reported.
Previous methods have discussed chemical methods for measuring instantaneous UV radiation, as well as accumulated UV dose, such as in U.S. Pat. Nos. 4,255,665 and 2,949,880. U.S. Pat. Nos. 8,829,457 and 5,148,023 describe electrical devices connected to a display unit capable of monitoring UV dose. The lack of a mobile device interface to interact with the device makes it less accurate since it cannot use information such as the location and local time for correction. It also has no notion of real-time feedback to the user. U.S. Pat. No. 9,068,887 is derived from “Amini,” but additionally utilizes the knowledge of location (as obtained by the mobile device) to correct UV Index readings. Some drawbacks of the earlier references are that they fail to utilize detection of operating environment, or sensor orientation to correct UV Index readings. Other previous methods discuss using visible light to estimate UV exposure, such as U.S. Pat. No. 9,360,364. That disclosure does not explain how they estimate UV exposure based on visible light, an arduous task since UV exposure does not correlate with visible light. A prime example of this lack of correlation is overcast weather, where visible light and heat are reflected/scattered by the clouds, but UV is still largely transmitted. The disclosure in U.S. Pat. No. 9,360,364 would be inaccurate in such situations.
Real-Time Notifications
U.S. Pat. No. 6,426,503 and US20040149921 describe providing touch-based feedback (using vibration), or with an audible alarm, when safe exposure thresholds are reached. It does not use notification on the mobile device. U.S. Pat. No. 9,068,887 describes notifications on a mobile device, but requires the wearable to be constantly connected (wirelessly) to the mobile device.
Separation of UVA and UVB
US 20120241633 describes a method to measure UVA and UVB separately. This is a pure hardware method involving the use of a photodiode with either a UVA filter or a UVB filter. The described hardware allows only one type of a measurement at a time.
User-Selectable Safe Thresholds
U.S. Pat. No. 9,068,887 describes “user-programmed safe thresholds”, which means users are able to select the safe amount of UV exposure that they are open to receiving. They use skin type information to select a default threshold for each skin type. The medical literature shows, however, that every person has a unique threshold. An exemplary system attempting to perform such measurements has previously been proposed in Amini. It includes a wearable device with sensors, which wirelessly communicates with a mobile device (such as a smartphone or tablet).
Improved methods, devices and systems are needed to overcome shortcomings of the approaches described above.